1. Field of the Invention
This invention relates to a process and system for production of methanol. More specifically, the invention includes a process and a system for producing methanol from combining a synthesis gas containing inerts, notably, nitrogen with a hydrogen-rich gas stream, and subsequently converting the hydrogen-rich synthesis gas stream to methanol, or other hydrocarbon chemicals. Even more specifically, the invention relates to such processes and systems capable of being mounted and operated on seagoing vessels, as well as on land.
2. Description of the Related Art
The evolution of methanol synthesis started at the beginning of the 20th century and reached commercialization in the mid 1920s. The reactants for methanol synthesis were H2, CO, and CO2, and this mixture was named “synthesis gas” or “syn gas.” A stoichiometric mixture when passed through a catalyst bed, only reacted 12% of these reactants because the reaction was stopped by reaching equilibrium with the methanol generated by the reaction. The industry adopted the use of a recycle system, known as a “methanol loop,” as a solution. The reacted gases from the catalyst bed were cooled, methanol condensed out, and the remaining gases re-circulated through the inlet to the catalyst bed. This system had the advantage of absorbing the exothermic heat generated at the catalyst site and carrying the heat to external heat exchangers. Up to 95% of the reactants were converted to methanol by this technique. This has been used in virtually all development of methanol synthesis during the last 80 years.
There are disadvantages of this system. The methanol loop placed a restriction on the purity of the syn gas. Consequently, inerts fed into the methanol loop with the syn gas had to be purged from the loop just after the methanol was condensed out. The purged inerts were intimately mixed with the valuable reactants, which were lost when purged along with the inerts. The prior art solution to this problem was to reduce the amount of inerts to as little as possible during syn gas preparation.
Modern practice in high capacity plants is to use an autothermal reformer (“ATR”) to prepare the syn gas. The device mixes an oxygen containing gas stream with a natural gas stream to partially oxidize the natural gas in the top of the reformer vessel. The lower portion of the reformer vessel contains a catalyst, which brings the oxidized gases into chemical equilibrium. The major ingredient in natural gas is methane, which is converted to syn gas and H2O.
Use of an ATR is also problematic because inerts are introduced into the natural gas stream along with the oxygen. To combat this problem, a cryogenic air separation (“C-ASU”) was utilized to produce oxygen with the lowest possible inerts, generally between about 1% to 5%. The majority of the inerts were N2 and Ar. Currently, there are no viable alternative processes for producing oxygen at large capacity and purity in this range. Cryogenic air separation is a difficult process to operate, has high maintenance and has a history of catastrophic explosions. Additionally, the CH4 in the reformer vessel outlet acts as an inert in the methanol loop. Therefore, furnaces and reformers were operated at temperatures on the extreme upper limits of metal and ceramic materials to minimize the CH4 remaining in the syn gas. For these reasons, the designs were expensive and consumed up to 80% of the overall plant energy.
Over the last 80 years, this approach has taken its toll in capital costs of the synthesis gas production portion of methanol plants.
COST BREAKDOWN BY PLANT SECTIONSyngas preparation 60%Methanol synthesis loop10Methanol distillation10Utilities20100%
Prior art methanol production processes include those manufactured and sold by Lurgi AG of Frankfurt, Germany. Such prior art Lurgi systems have been disclosed, for example, in European Patent 0790226 B1, U.S. Pat. No. 5,631,302 and U.S. Pat. No. 5,827,901, which are hereby incorporated by reference in their entireties. Similar systems utilizing a methanol loop include those used by ICI and Holder-Topsoe, including those systems described in U.S. Pat. Nos. 6,387,963 6,881,759, 6,730,285, 6,191,175, 5,937,631 and 5,262,443, and United Kingdom Patent Nos.: 1,159,035 and 1,272,798, which are all hereby incorporated by reference in their entireties.
Synthesis gas has also been manufactured from oxidant streams high in nitrogen, such as air. Such processes have used separation processes, such as semi-permeable membrane technology, to separate air streams into high oxygen content streams and high nitrogen content streams. The high oxygen content streams were then reacted with natural gas to create synthesis gas, which was then converted to methanol.
Special reaction systems had to be developed because the high nitrogen content in the synthesis gas stream created problems in conventional methanol production processes by limiting the yield and the effectiveness of the methanol reactors. Such processes are disclosed in U.S. Pat. Nos. 5,472,986 and 7,019,039, which are hereby incorporated by reference in their entireties. These patents are assigned to Starchem Technologies, Inc. and the methanol production processes described therein are generally referred to herein as the “Starchem system.” In the Starchem system, a reactor recycle stream (methanol loop) was not used because of problems associated with the high nitrogen content. As such, a series of single pass reactors were required.
Similarly, European Patent Application 0 261 771 proposed the use of air for production of a high nitrogen content synthesis gas which, thereafter, would be processed through a series of plug flow methanol reactors with interstage removal of methanol and water. As such, a series of single pass reactors were required, just as in the Starchem system.
The ATR and the methanol loop are not compatible without modifications. This can be explained in terms of the stoichiometric number (“Ns”) defined as Ns=(H2−CO2)/(CO+CO2).
Ns is commonly used as a measure of how syn gas will perform in the methanol loop. A number greater than 2 indicates an excess of hydrogen over that required for conversion of all the carbon to methanol. A number less than 2 indicates a hydrogen deficiency. The methanol loop may become in-operable when deficient in hydrogen. Make up gas (“MUG”) is the name of the gas injected into the methanol loop. Experience has shown that a MUG with Ns=2.05 produces the most efficient and lowest capital cost methanol loop design.
A characteristic of an ATR is the reformed syngas has an Ns of about 1.75. The traditional approach has been to add hydrogen to the effluent of the ATR to increase the Ns of the MUG stream to about 2.05. The source of hydrogen has been from fired steam methane reforming or from refineries. More recently, the Ns has been increased by rejecting CO2 from the gas mixture as in Starchem's U.S. Pat. No. 7,019,039 for a series of single pass reactors.
It would be desirable to utilize prior art methanol production systems that include a methanol loop, with a synthesis gas produced through partial oxidation of natural gas using an air stream, such as the Starchem system. U.K. Patent Application 2,237,287A and Australian Patent AU-B-6459390 (“the AUS Process”) describe the use of a synthesis gas formed from an oxygen enriched gas stream for the partial oxidation of natural gas and the use of a methanol loop reactor system for methanol production. In the AUS Process, a portion of the synthesis gas is not subject to the methanol synthesis loop. Rather, the synthesis gas is split into two distinct streams, stream “A” and stream “B,” upon leaving the ATR. The “A” stream is diverted to a water gas shift reactor, converting CO to H2, and then through a pressure swing absorber (“PSA”) to extract the H2. The H2 that has been extracted joins the “B” stream and the combined flow is a hydrogen enhanced syn gas. This method is less than desirable because of the need for additional equipment for the extraction of the H2 and the diversion of some of the synthesis gas, which results in a reduction in potential methanol production.